My boats voltage low alarm keeps going off. Normally on idle when I have the engine on the volt meter reads 13 volts and when i give it throttle it reads like 14. Now its at like 11 at idle and 12 at wot. Is this the alternator? Also Where can I buy a replacement alternator and can i install it myself? Are there any alternators designed for saltwater? My boat is a mastercraft with the indmar rtp-1